Synthesis of stereodefined vinyl-tetrahydropyran and vinyl-octahydrochromene derivatives via acetalization–cyclization of allylsilanes with aldehydes. Origin of the high stereoselectivity

Functionalized allylsilanes 1–5 and aldehydes 6–8 undergo Lewis-acid mediated ring closure to afford 2,3,5- or 6-substituted tetrahydropyrans (8–15) and 2,3-substituted octahydrochromenes (16a–b) with excellent stereoselectivity. According to DFT calculations the high stereoselectivity arises from electronically induced steric effects occurring in the key-intermediate of the cyclization.